Whence the Shadows? Ill 



fish is there, at least at first glance. But, demonstrating in still another 

 way its tendency to remain basically simple, the shark has a skeleton 

 that differs considerably from that of the bony fish. Without going into 

 anatomical detail, it may be said that the Teleost's skull is a far more 

 complex bony structure than the Selachian's cartilaginous skull. 

 / ' The skin of fish, like the skin of man and other vertebrates, consists 

 of an epidermis, an outer layer of cells, which is continually wearing 

 away and being replaced, and the dermis, an inner layer of more com- 

 plex cells which include the pigment cells that determine color. Gen- 

 erally, the skin of fish is covered with scales, and most fish scales are 

 of two types: cycloid scales, found in such fish as carp and herring, and 

 ctenoid scales, which have minute spine-like projections at their exposed 

 edges (a black bass has ctenoid scales). Sharks have a third type of 

 scales— placoid. And these scales are really dermal teeth, set in the shark's 

 hide. 



Of all the many oddities of the shark, this is one of the most difficult 

 to grasp, perhaps because it is so uncomplicated. These scales, called 

 dermal denticles, are truly teeth. Each denticle in the shark's hide has 

 the two attributes of a tooth: its surface is covered by dentine, and it 

 has a central pulp canal containing a nerve and blood vessels. In some 

 species, these denticles are visible to the naked eye; in other species, they 

 are microscopic. But, no matter the size, they are teeth. The denticles 

 give the tough hides of most sharks a sandpaper-like roughness that can 

 scratch or even tear a swimmer's flesh. This abrasive hide, called shagreen, 

 can smooth down the hardest woods and, in fact, was once used for that 

 purpose by cabinetmakers, as has been mentioned. 



Denticles are anchored in the skin of the shark much as collar but 

 tons are held in a shirt. The sub-surface base of the denticle is larger 

 than the opening through which the visible portion projects. The denti- 

 cles project backward, which is very obvious if the skin is stroked from 

 the tail toward the head. In some species, such as the Nurse shark 

 (Ginglymostoma cirratum), the denticles are so large and so closely 

 spaced that it is difficult to drive a harpoon into the hide. Other species 

 produce scattered patches of denticles. The variety of denticle forms 

 is nearly as great as the variety of shark species. Denticles are blunt, 

 scalloped, spade-shaped, thorn-like, geometric, and even heart-shaped. 



By a growth process called hypertrophy, certain denticles develop 

 independently of others and become comparatively gigantic structures 

 with no apparent relationship to the smaller and microscopic denticles. 

 The possession of denticles is one of the many characteristics shared by 

 sharks, skates, rays, and the links between them. 



These other members of the Selachian family are usually so segre- 

 gated from the sharks themselves in most writings about sharks that it is 



